One component arrangement is described, by way of example, in DE 102 32 642 A1. FIG. 1 shows one such known component arrangement in the form of a side view (FIG. 1a), a plan view of the planar windings (FIG. 1b), and in the form of an electrical equivalent circuit (FIG. 1c).
In this component arrangement, a dielectric layer 120 is arranged on a semiconductor body 110 and electrically isolates a primary winding 140 and a secondary winding 130 of a planar transformer from one another. The secondary winding 130 is connected, for example, to integrated circuit components (which are not illustrated in any more detail) in the semiconductor body. The primary winding may be connected to other circuit components in the same semiconductor body 110 or in another semiconductor body (not illustrated). The circuit components to which the primary winding 150 is connected form, in particular, a transmission circuit, and the components to which the secondary winding is connected form, in particular, a receiving circuit for a data transmission device, in which the transformer is used as an inductive coupling element between the transmitter and receiver, and at the same time as a potential barrier between the transmitter and receiver.
The primary winding 140 and the secondary winding 130 are each arranged as a conductor loop with two or more turns on in each case one (metallization) level in the dielectric layer 120, and thus form a planar transformer without a transformer core, which is referred to in the following text as a coreless transformer.
In the equivalent circuit shown in FIG. 1c, C140 and C130 denote the capacitances of the primary winding 140 and of the secondary winding 130, which in each case act between connections 140_1, 140_2 and, respectively 130_1, 130_2 of the respective winding 140, 130. R140 and R130 denote the resistances of the primary winding 140 and of the secondary winding 130, and L140 and L130 denote the inductances of the primary winding and of the secondary winding 130. The coupling factor k between the primary coil is less than unity, k·L140 denotes the coupling inductance on the primary side of the transformer in the equivalent circuit, and k·L130 denotes the coupling inductance on the secondary side of the transformer. (1-k)·L140 and, respectively (1-k)·L140 denote the stray inductances, which are dependent on the coupling factor. Csub/2 denotes parasitic capacitances in FIG. 1c, which result from any capacitive coupling between the secondary winding 130 and the semiconductor body.
Parasitic effects also result in capacitive coupling between the primary winding 140 and the secondary winding 130. C134/2 in FIG. 1c denotes the parasitic capacitances which result from this and respectively occur between one of the connections 141_1, 141_2 of the primary winding 140 and one of the connections of the secondary winding 130.
Coreless transformers of the type explained above are used, for example, in half-bridge circuits for the transmission of a drive signal from a control circuit to a high-side switch in the half-bridge circuit, in order to decouple the potentials in the drive circuit and in the high-side switch. In circuit arrangements such as these, electromagnetic interference signals occur during switching processes of the high-side and low-side switches which form the half-bridge circuit and are normally in the form of power transistors, and these interference signals can induce interference voltages in the windings of the transformer. These interference voltages are produced by displacement currents in the parasitic capacitances between the primary winding and the secondary winding and may, in some circumstances, reach the level of useful signals to be transmitted.
In conventional iron-core transformers, which have been known for a long time, the effect of parasitic capacitances is reduced by the use of a shielding layer between the primary winding and the secondary winding of the transformer.
In so-called pulse transformers, which are used for signal transmission, the primary winding and the secondary winding are arranged as far apart from one another as possible on a torroidal annular core, although this does not significantly reduce the parasitic capacitances, since, as before, there is still a large capacitance between the windings and the annular core.
Differential transmission methods are known for signal transmission using planar coreless transformers, and these allow detection of interference signals which are injected into the transmission path. Methods such as these are described, by way of example, in DE 102 29 860 A1. These transmission methods are, however, comparatively complex.